# Heartbeat (35=0)

The Heartbeat message `(tag 35=0)` is used by both the initiator and the National Digital Asset Exchange (NDAX) to maintain the active status of a Financial Information eXchange (FIX) session during periods of inactivity. The duration of the inactive period is determined by the `HeartBtInt field (tag 108)` in the Logon message `(tag 35=A)`. The Heartbeat message is sent by the initiator and NDAX at regular intervals during the FIX session to confirm that the session is still active and to ensure that the connection between the two parties has not been lost. If a Heartbeat message is not received within the designated inactive period, the session may be terminated. The Heartbeat message is an important feature of the FIX protocol that helps to ensure the reliability and continuity of the FIX session.

<table><thead><tr><th width="86">Tag</th><th width="124">Field Name</th><th width="121">Data Type</th><th width="75">Req’d</th><th>Description</th></tr></thead><tbody><tr><td>112</td><td>TestReqID</td><td>String</td><td>N</td><td>Conditionally required if the heartbeat message is generated in response to a <code>TestRequest (35=1)</code> message.</td></tr></tbody></table>


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